Eratosthenes measured the radius of the Earth with an incredibly accuracy.
To do it, you need to measure the length of the shadows from 2 different cities at the same time of the day. Then knowing the distance between the cities and a little bit of geometry he calculated the radius of the Earth.
My question is: How did he know when it was "the same time of the day"? There was no long range communications to synchronize two people measuring the shadows.
And if you measure the shadow one day, then travel to the other city to measure the shadow next day, how can you know that you will do it at the same time of the day?
Nowadays with clocks or telephones the experiment is very easy, but back then I can not figure out how he managed to do it.